Examples of hydraulically-actuated fuel injection systems are shown in U.S. Pat. No. 4,448,169 issued to Badgley et al. on May 15, 1984 and U.S. Pat. No. 4,459,959 issued to Terada et al. on Jul. 17, 1984. Engines equipped with a hydraulically-actuated fuel injection system will not start quickly unless the system is able to quickly pressurize the actuating fluid used to actuate the fuel injection system. A noticeable delay in the startability of an engine can be annoying to the engine operator or unattractive to potential engine buying customers.
The hydraulically-actuated fuel injection system typically includes a high pressure actuating fluid pump which is driven by the engine. Because the pump speed has a fixed relation to engine speed, starting fuel requirements must be met at very low engine speeds while the engine is cranking under auxiliary power from a battery and starter motor. The pump loses efficiency at the relatively low cranking speed and therefore its pumping capacity must be large enough to compensate for that. Also the pumping capacity must be large enough to compensate for leakages of actuating fluid in the system.
For the above reasons as well as instances where an engine requires overfueling during startup, the cranking fuel delivery requirement determines the maximum pump size or capacity required for the engine. This means the pump will be of excess capacity or oversized at other speeds of the engine such as rated speed and load. This can be a significant efficiency loss in a system having a fixed displacement high pressure actuating fluid pump.
The present invention is directed to overcoming one or more of the problems as set forth above.